1. Field of the Invention
The present invention relates to a method for forming fine patterns on semiconductor devices, and more particularly to reducing the size of pattern features through post-development processing.
2. Description of the Related Art
Demand for semiconductor devices having increasing processor speeds and memory devices having ever higher integration densities for use in a wide range of electronic devices has led to a corresponding demand for production of circuit features of sub micron sizing. Lithographic developments, such as improved photoresist compositions and improved dimensional accuracy of the resulting patterns, particularly for structures of minimum feature size, have been driven by these demands for increased integration density. The formation of highly accurate fine photoresist patterns which are widely used to mask etch and ion implantation processes is required for successfully manufacturing high density semiconductor devices. A sensitive photoresist is needed to define these features, but the use of sensitive photoresists presents additional process complications. It is anticipated that that ArF lithography may be utilized in the fabrication devices manufactured using a 0.13 xcexcm process using a single layer resist, but that even greater accuracy and dimensional control will be needed for future sub-0.10 xcexcm processes.
According to Rayleigh""s equation
R=k1xcex/NA
wherein R denotes the ultimate resolution, k1 is a constant, xcex is the wavelength of the light source used during the exposure, and NA is the numerical aperture of the illuminating optical system, decreasing the wavelength of the exposing light source will tend to enhance the ultimate resolution. This principle has been applied during the transition of photolithographic processes from g-line (436 nm) to i-line (365 nm) exposing light sources and more recently to projection exposure equipment using KrF excimer lasers (248 nm) as the exposing light source. This trend toward ever smaller wavelengths is leading to development of projection exposure equipment and processes that may utilize an ArF excimer laser (having a central wavelength of 193 nm) or an F2 excimer laser (having a central wavelength of 157 nm) as the exposing light source.
The wavelength of the exposing light source directly impacts the minimum resolution that may be obtained on any piece of exposure equipment. For instance, in the creation of fine line and space (L/S) patterns, the resolution limit of g-line exposure equipment is approximately 0.5 xcexcm and the resolution limit for i-line exposure equipment is approximately 0.3 xcexcm. More recent device design rules, however, are tending toward L/S measurements of less than about 0.2 xcexcm. In order to transfer such patterns from projection masks onto wafers with sufficient accuracy, KrF and ArF excimer lasers have been utilized as the exposure light sources.
It is anticipated that the minimum feature sizes allowed by future device design rules will continue to decrease. Candidates for exposure apparatus capable of manufacturing devices built to such design rules include F2 excimer laser, X-ray and electron beam (EB or E-beam) exposure apparatus. In particular, as device design rules allow for smaller and smaller minimum features, it becomes more difficult to provide sufficient process margins for the formation of contact and via holes than for the formation of L/S structures having similar sizing. This is particularly true for high-density devices that require the formation of small contact holes with high aspect ratios in the cell array region of the device.
Various techniques have been developed to address the difficulties inherent in forming small, high-aspect ratio contact holes including, for instance, thermal flow and the RELACS (Resolution Enhancement Lithography Assisted by Chemical Shrink) processes. The thermal flow process forms a fine pattern by forming an initial contact hole photoresist pattern with contact holes sized larger than the desired final contact hole size, and then heating the photoresist pattern to a temperature above the glass transition temperature (Tg) of the photoresist. This heating reduces the viscosity of the polymerized photoresist and allows it to flow or slump, thereby reducing the size of the contact openings to achieve the desired contact hole sizing.
Although thermal flow processes may be useful in photolithographic processes utilizing i-line or KrF resists, in the case of ArF resists the Tg (a minimum of approximately 200xc2x0 C.) is generally higher than their decomposition temperature (Td), typically not more than approximately 180xc2x0 C. Therefore, the thermal flow process cannot be used with ArF resists because the resist pattern will begin to decompose before it flows.
The RELACS process forms a fine pattern by again forming a normal contact hole photoresist pattern with contact openings sized larger than the desired final contact hole size. This initial photoresist pattern is then coated with a water-soluble polymer that reacts to form an insoluble cross-linked layer along the surface of photoresist pattern. The unreacted polymer is then removed by rinsing the photoresist pattern. The cross-linked layer increases the effective size of the photoresist pattern, thereby reducing the size of contact openings or the spaces in L/S structures. RELACS processes utilizing a single step for removing the water-soluble polymer, however, may be subject to problems of incomplete removal resulting in development residues, such as specks or films, in the pattern. During subsequent etch processes, such development residues increase the chance of defects in the final device, depressing yield and reliability. A two-step cleansing process in which the wafer is cleaned with a first solution and is then rinsed with water may decrease the amount of development residue left on the wafer, but also tends to complicate the process and increases the expense.
Resist compositions suitable for use with ArF excimer laser exposing light sources (ArF resist) such as poly(meth)acrylate, cyclo-olefin maleic anhydride (COMA) and polynorbornene, are susceptible to problems with critical dimension (CD) line shrinkage during SEM measurement (also referred to as CD slimming) as shown in FIG. 1. ArF resists also suffer from weak dry-etch resistance, Line Edge Roughness (LER) and CD slimming. LER and CD slimming in particular remain significant issues in processes utilizing ArF resists. During CD-SEM measurement, the electron bombardment of the fine resist features during measurement can actually permanently reduce the feature size. If the same resist feature is re-measured, the feature size will continue to shrink. An example of how the measured CD decreases during repeated measurements is shown in FIG. 2.
The differences in composition between KrF resists and ArF resists may produce significant differences in the performance of the materials. For example, KrF resists are more resistant to CD slimming ( less than 2%) during CD-SEM measurement while ArF resists exhibit CD slimming of 6% to 15% during SEM measurement. These results suggest that CD slimming could be a widespread problem with ArF resists. Generally, the CD of the patterned lines tends to decrease while the size of the corresponding patterned space or hole tends to increase, a particular problem for minimum dimension openings. Additionally, ArF resist patterns may exhibit undesirably roughness (LER) as a result of decreased homogeneity of the ArF resist during polymerization.
Recently, E-beam curing has been proposed as a possible remedy for the CD slimming and LER problems experienced with ArF resists. In E-beam curing, the ArF resist pattern is hardened before measurement to reduce CD shrinkage during SEM measurement, typically by exposing the developed ArF resist pattern to a number of E-beam doses using a focus expose matrix (FEM) to increase the degree of crosslinking and rigidify the resist pattern before significant decomposition or mass loss occurs. E-beam curing, however, increases the complexity and length of the manufacturing process and the additional handling increases the likelihood of particulate contamination.
Exemplary embodiments of the invention provide methods for forming fine patterns, particularly contact holes, on semiconductor devices by forming an ArF resist pattern and then reducing the size of pattern openings by exposing the resist pattern to radiation from an VUV (vacuum ultraviolet) excimer laser having a wavelength of not more than 172 nm or E-beam radiation during thermal treatment. VUV refers to a region of the electromagnetic spectrum between the longest-wavelength X-rays and far ultraviolet covering a wavelength range of approximately 100 to 200 nm. Light energy in this range of the spectrum is attenuated by absorption in O2, requiring most VUV processes to be carried out in a vacuum or under N2 purging to reduce the atmospheric O2 content, hence the name. In particular, the exemplary embodiments of the invention provide methods for shrinking the size of contact hole by exposing an ArF resist pattern during thermal treatment at a temperature between about 30xc2x0 C. and about 180xc2x0 C. to VUV light energy or E-beam energy to cause the resist pattern to flow in a controllable and reproducible manner. This patterning method thereby provides the ability to reduce the patterned opening sizing in fine resist patterns while improving CD stability and reproducibility to enhance the process yield and the reliability of the resulting semiconductor devices.